This invention relates to a gas humidification system.
Although the system of the present invention may be used in a variety of environments, without limitation it is discussed herein in the context of fuel cells. Fuel cell technology is being further and further refined with a view toward its replacement of common, environmentally destructive power sources such as the internal combustion engine.
Although there are several types of fuel cells, the present invention is primarily related to proton exchange membrane or polymer electrolyte membrane (PEM) fuel cells, the type of cells currently thought to be best suited for transportation applications such as passenger vehicles. In PEM fuel cells, two electrodes—an anode and a cathode—are separated by a membrane that functions as an electrolyte. The electrolyte permits protons to pass through the membrane while blocking electrons from flowing through the membrane. Catalysts are provided in the anode and cathode sides of the cell to help cause this electrochemical reaction.
The reaction occurs when molecules of hydrogen gas (H2) enter the anode side of the fuel cell. The anode, together with the catalyst on the anode side of the cell, repels the electrons of the hydrogen molecules. In addition, the electrolyte membrane prohibits passage of electrons therethrough. As a result, the electrons of the hydrogen molecules travel around the membrane by means of an external electrical circuit as the protons of the hydrogen molecules travel through the membrane. The stream of electrons through the external electrical circuit is utilized as an electrical power source. On the cathode side of the cell, oxygen or an oxygen-containing gas is provided. After the hydrogen electrons (H−) reenter the fuel cell and the hydrogen protons (H+) pass through the membrane, the cathode and the associated catalyst cause the hydrogen electrons and protons to react with the oxygen in the cathode side of the cell to produce water (H2O).
Many technical problems have arisen as PEM fuel cells have been developed and tested in various contexts. One such problem is membrane dryout. Specifically, if the membrane drops below a certain humidity, ion conduction through the membrane is reduced, thereby reducing cell efficiency and eventually causing cell failure. This dryout occurs as a natural result of cell operation and will not be remedied unless a system for accomplishing membrane humidification either accompanies or is incorporated within the cell.
Some inventions addressing the membrane dryout problem employ flow-through plates or membranes to transport water or other humidifying liquids to the fuel cell membrane. For instance, U.S. Pat. No. 4,769,297 to Reiser et al., U.S. Pat. No. 5,064,732 to Meyer, and U.S. Pat. No. 5,853,909 to Reiser disclose porous humidification plates situated between adjacent cells in a fuel cell stack for transporting product water from the cathode of one fuel cell to the anode of an adjacent fuel cell to achieve humidification. However, these products require maintenance of a constant pressure differential across the humidification plate to achieve the desired water flow. The sudden, transient pressure increases that periodically occur in these systems exacerbate the problem of pressure differential regulation. In an effort to remedy these concerns, pressure regulators and related devices are frequently incorporated into the fuel cell stack design; construction and maintenance costs, in addition to overall failure probability, are increased by the addition of this machinery.
U.S. Pat. No. 5,382,478 to Chow et al. discloses a humidification assembly including a large plurality of humidification cells, each cell being formed by a water vapor transport membrane between two flow field plates. Although pressure differential maintenance is less of a concern in this design, the complexity of this humidification assembly increases the cost and failure probability of the fuel cell assembly.
Earlier fuel cell designs, such as that disclosed in U.S. Pat. No. 3,061,658 to Blackmer, include water reservoirs with membranes or other absorptive materials carried therein to facilitate a wicking, capillary action of water into reactant gases passing over the reservoirs before entering the fuel cell. Such designs are simpler and less costly than those discussed above; however, because the absorptive materials are not sealed at their edges to form a semi-permeable barrier between the water source and the reactant gases, pressure differentials between the gases and the water can cause flooding of water into the gas passages or bubbling of water in the reservoir, thereby causing overhumidification or underhumidification, respectively, of the gases and, by extension, the fuel cell membrane. Designs employing wicks in fluid communication with reservoirs, such as those disclosed in U.S. Pat. No. 3,507,702 to Sanderson and U.S. Pat. No. 3,677,823 to Trocciola, experience similar problems.